1. Field of the Invention
The present invention relates to a process for forming a CVD (chemical vapor deposition) film and a semiconductor device, and more particularly to a process for forming an insulating film such as an SiO.sub.2 film according to a CVD method and a semiconductor device using such a CVD film.
The multi-layer wiring technology has been in progress as a powerful means for increasing the scale of integration of semiconductor devices. In keeping with this, development of a method of forming an interlayer insulating film which is most adaptable for a multi-layer wiring structure has been increasingly in demand. In order to eliminate disconnection of metalic wirings because of stress migration and minimize the strain of devices and the like because of thermal stress, there has recently been a demand for development of a process capable of forming an interlayer insulating film at as low a temperature as possible.
2. Prior Art
Various proposals have heretofore been made of processes for forming an SiO.sub.2 film according to a chemical vapor deposition method (hereinafter referred to as a "CVD method"), among which representative processes capable of forming an SiO.sub.2 film at a low temperature in particular include those mentioned below.
(1) SiH.sub.4 --O.sub.2 Process
This is a process for depositing an SiO.sub.2 film on a semiconductor substrate at a temperature of 350.degree. to 450 .degree. C. according to a reaction represented by the following formula: SiH.sub.4 +SiO.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2. This reaction occurs in a gaseous phase, and hence an SiO.sub.2 film is formed through simple deposition.
According to this process, a large number of particles are liable to be generated to deteriorate the quality of the resulting film and liable to give rise to uptake of moisture into the film, presenting a problem of poor reliability. Furthermore, the poor step coverage of the film entails a problem of disconnection of wirings formed on the film. Moreover, special care must be taken of SiH.sub.4 as a reactive gas to be used in the reaction because of its self-burning properties. This makes the handling of SiH.sub.4 complicated.
(2) Plasma CVD Process Using SiH.sub.4 -N.sub.2 O or Like
This method is advantageous in that an SiO.sub.2 film can be formed at a temperature as low as 300.degree. to 400 .degree. C. However, since it is a film formation process utilizing charged particles, damage to a device is so great that the process is unsuitable for use in production of very fine devices and VLSI structures. Furthermore, plasma discharge is so liable to break down all kinds of chemical bonds that a good-quality film can not always be obtained. Moreover, the process involves generation of a large number of particles and a great possibility that metallic atoms and the like constituting the inside of a reaction chamber used in the process may be incorporated into the resulting film to cause contamination of the film. In addition, the film obtained by the process has a problem of poor step coverage.
(3) TEOS-O.sub.3 Process
This is a process wherein TEOS (tetraethyl ortho-silicate, Si(OC.sub.2 H.sub.5).sub.4, an alkoxysilane) is decomposed with the aid of O.sub.3 (ozone) to form an SiO.sub.2 film.
This method has various advantages such as safety of TEOS, good step coverage, little generation of partices, and other merits in association with the reaction rate determination of surface. Accordingly, this process can eliminate the defects of the above-mentioned SiH.sub.4 -O.sub.2 process (1) and plasma CVD process using SiH.sub.4 -N.sub.2 O and the like (2).
In accordance with the process for forming an SiO.sub.2 film according to the TEOS-O.sub.3 reaction, however, the deposition rate of an SiO.sub.2 film is dependent upon the O.sub.3 concentration, while the film quality thereof is not always satisfactory and the step coverage of the film around stepped portions of the underlying surface turns from flowing profiles into isotropic profiles as the O.sub.3 concentration is decreased (about 3 mol % or less). This entails a difficulty in using such SiO.sub.2 films as interlayer insulating films.
Furthermore, when the underlying surface is of a thermal SiO.sub.2 film, the unevenness, or roughness, of the surface of an SiO.sub.2 film formed thereon by the TEOS-O.sub.3 process increases with an increase in the O.sub.3 concentration to increase the possibility of causing failures such as disconnection of wirings.
Moreover, the TEOS-O.sub.3 reaction involves a high possibility of particle generation because it is not necessarily a perfect surface reaction.
Thus, in accordance with the process for forming an SiO.sub.2 film according to the TEOS-O.sub.3 reaction, the film quality, the step coverage, the film surface state, the film deposition rate, etc. are so dependent upon the O.sub.3 concentration that SiO.sub.2 films cannot always be formed stably.
The present invention has been made in view of the foregoing problems of the prior art. Accordingly, an object of the present invention is to provide a novel CVD process easy of production control with a constant deposition rate and capable of forming a good-quality CVD film with good step coverage and other characteristics.